Twin-engined boats - a question? *DELETED*

[Guest]

Re: Spoiler

Byron,

thanks for the suggestion, Tosca Too, the other half saw the post liked the idea and now, unless alresday registered, it will be Tosca Too as opposed to Tosca2.

 

[boatone]

Re: ToscaToo at Chichester........

Sounds almost operatic and definitely more upmarket than Tosca2 at Ramsgate.
Matts and Byron will await your boatwetting ceremony invitations................

TonyR
boatone@boatsontheweb.com

 

[Guest]

Re: ToscaToo at Chichester........

and once I have bought a big enough lumps to tow it around I can have Tosca Too an tour

 

[Guest]

Re: Boats and planes

Umm mumble that was my silly idea. With you having a plane and a boat. And me not being bothered to guess, so started making you guess, evidently with some success.

dare I ask you to get rid of that signature? Quite good first time but yuk after a few times round praps bit like cult sixties tv series....

As for boat: They should have invoices and everything and contacts to explain exactly what has been done. If they haven't then discount to assume that it isn't properly balanced and so on. But the sea trial should establish how it feels. I mean, did it thrumm nicely or was it an utter boneshaking boatwobbling racket?

 

[BarryD]

Alpha - Do you pprune by any chance - your signature looks related? If so what ID do you use?

 

[alpha]

Re: Boats and planes

OK, The signature has gone (proving its truth in the proces!!)

Of course, we're looking into past work done, and are already part way down that path, though you'll know that keeping invoices neatly in a binder is not something that comes easily to all <grin>. In answer to your question, it thrummed quite nicely, though with a definite out-of-synch beat. Perhaps it's just a matter of getting used to it?

The fact is, she'll be mostly used on the river, and is vastly over-powered for that. Our present boat (much smaller, and Pauline knows what make), runs so sweetly and smoothly, that the thrumming was a bit of a shock.

Thanks for the replies so far.

 

[byron]

Whilst I do not have a definative answer I do know that some boats have different prop size/pitch on twin screws. A notable example of this is on the early Oceans/Contintentals/Crowns. I believe it's something to do with torque and running the engines at the same revs while keeping a straight line.

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www.education-jobs.co.uk
www.alexander-advertising.co.uk

 

[kimhollamby]

In some cases the different prop sizes are to do with the fact that, in order to get contra-rotation, some boat manufacturers had to run one gearbox permanently in what the gearbox manufacturer intended as astern gear and the forward and astern ratios are, belive it or not, different. With older boats there's always the additional risk that a gearbox has been replaced and changed for one of different ratio - have seen at least one example of that too.

 

[Guest]

Re: understanding more

ah, I see. The gears are different ratios...so the props should be different to match...and if they aren't then there's that out of synch wah-wah-wah, and even if they are the props are sort-of not balanced against each other...

much better without signature, cheers, but freedom acknowledged. You are not a number - you are a lettter of the alphabet!

 

[jfm]

Re:Engineeringy reply

Alpha, I have not had such a boat but do possess a dodgy degree in mech eng from exhibition road and I wd think that diff gearbox ratios then diff prop torques would be a risky idea. It's easy to do, but not commonly done, therefore likely a bad idea.

You don't say whether its a big difference or a marginal one. I missed your orig post. If a marginal difference, why bother. I wd suspect that a gearbox has been replaced and the prop pitch adjusted to compensate. Not good, but you cd live with it. If a massive difference, you will get quite different prop blade speed thru the water and this will affect performance, I mean one or other prop could be sub optimal.

Finally there could be funny noise effects. Both engines are same rpm so no funny harmonics there. But props make a noise too and will be going different rpm so that could result in funny harmonics, but could only test that by driving the boat

I do not think it's anything to do with reverse gear unless very old. Most marine gearboxes these days have identical reduction ratio in forward as reverse. Reason is that there is no intrinsic fwd and reverse, they're mechanically indifferent. On a normal contra-rotate twin shaft drive boat, one gearbox is run "in reverse" and that is how the contra-rotate is achieved. The engines obviously both spin the same way. Hence reverse and fwd ratios are made the same

Sorry, as a matter of principle I will not accept any conditions imposed on not drifting the thread. Sacred Cow I'm afraid (or, "that's what you Brits call a holy cow" as an American visitor staying with us once said).